A suitable device is needed for the direct measurement of oxygen partial pressure (P02) in blood and tissue for both clinical and research applications. Methods currently available for measuring P02 lack convenience, reliability, speed, and relevance to many situations of interest. Efforts to develop electrical sensors have not been successful. It is desirable to have a very small P02 sensor which can be inserted into a blood vessel or tissue with little disturbance, and which will provide instantaneous and current P02 monitoring for either short or extended periods of time. A fiber optic sensor is ideal for this application, with the advantage, for physiological use, of very small size and flexibility, safety, and low cost. A P02 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. The feasibility of the sensor and its satisfactory performance has been demonstrated in in vitro and in vivo tests, in work in preceding years. In the past year, the development of the sensor as a needle probe was accomplished, and problems of storage life and stability were resolved. This provides a basis for practical application of the fiber optic P02 probe as a research tool.